The present invention relates to a method and a device for automatically adjusting the ignition timing of a controlled ignition engine as a function of preselected operating conditions of the engine. Such conditions may for example, but not exclusively, be an optimum adjustment of the spark advance for maximum output and efficiency of the engine.
The operating conditions of the engine being known, for example by measuring the values of characteristic parameters (running speed of the engine, intake manifold vacuum, temperature and fuel-air ratio of the mixture which feeds the cylinders, proportion of burnt gases recycled to the engine intake, etc. . .), it is well known that under substantially normal operating conditions of the engine there is a relationship between the sparking time in a cylinder and the time at which the pressure rise in this cylinder reaches its maximum or peak value, these times being generally identified by the corresponding angular positions of the crankshaft.
Thus, for example, when the ignition timing corresponds to the optimum spark advance, it has been ascertained that the pressure peak in the considered cylinder occurs at a determined angular position of the crankshaft, about 15.degree.30' after the top dead center. The optimum spark advance can thus be obtained by adjusting the ignition timing until the observed pressure peak coincides with the above angular position of the crankshaft, i.e., substantially 15.degree.30' after the top dead center.
One problem to be solved is to determine with accuracy the instantaneous value of the pressure in the engine cylinders, to locate the angular position of the crankshaft corresponding to the maximum value of this pressure and to correspondingly adjust the ignition timing.
In French Pat. No. 2,109,698, it has been proposed on page 3, lines 14-16 and 34-36 "to so control the spark advance that the explosion always occurs in the optimum angular position of the crankshaft. This is achieved by using a sensor whose response curve is more or less linear, this sensor delivering a signal having a steep leading edge." A drawback of the process results from the fact that, in practice, there is no instantaneous explosion, so that the control of the spark advance is not accurate and this is particularly true as the engine running speed is increased.
In French patent application 2,270,450, reference is made on page 3, lines 24-31 "to prior studies on combustion in an internal combustion engine showing that this combustion comprises two different steps. The first step, which immediately follows sparking is a low pressure combustion step wherein the gas mixture is ignited and a flame begins to spread into the combustion chamber. Then a strong pressure discontinuity is observed corresponding to the beginning of the second combustion step." This beginning of the second step can be detected by using for example, a piezo-electric transducer which delivers a signal as soon as the pressure rises above a preselected threshold value and the spark advance is so adjusted that the beginning of the second combustion step occurs at a preselected angular position of the crankshaft. Such a process suffers from the drawback of requiring a substantial modification of the engines to permit location of the piezo-electric sensor and it appears difficult in practice to detect with a sufficient accuracy the beginning of the combustion step.
It has already been proposed, during the S.A.E. Meeting No. 750,883 (held in Detroit, Michigan, U.S.A., Oct. 13-17, 1975) to use ring-shaped piezo-electric sensors placed between the spark plugs and the cylinder head. Such sensors may work satisfactorily, but they make replacement of the spark plugs with new ones more difficult because this operation must be effected without damaging the sensors. Additionally, it is necessary to obtain proper sealing when positioning the new plugs. Moreover, the electric pulses generated by the ignition may in some instances interfere with the sensors' operation and make repair and maintenance operations in the plug vicinity more difficult.